1. Field of the Invention
The present invention relates generally to video decoder systems, and relates more particularly to compensating distortion in video decoders.
2. Description of Related Art
According to a known standard, television images are decoded with a quadrature amplitude modulated (QAM) technique that is based on the decomposition of signals into uncorrelated orthogonal components. The components are processed to contribute to producing a television image with color information included in the components. Ideally, a base band signal with two components, for example, is frequency shifted for transmission, resulting in side band frequencies that are symmetrical in nature, so that there is no cross-talk or channel interference between the components during QAM decoding. However, in practice, the side band portions are distorted through transmission, or due to mistuning or other performance issues in an intermediate frequency (IF) stage of a video receiver. Lack of symmetry in the frequency side bands produces cross-talk in the decoded image when the QAM components are recombined in producing the video image.
A decoded TV image passed through a mistuned IF stage produces noticeable visual artifacts due to interference or cross-talk between the different decoding channels. Mistuning introduced in the IF stage introduces excessive and asymmetrical attenuation in the upper side band of the QAM color difference signal representing a color difference from a reference color having a specified chromaticity. The distortion of the upper side band signal introduces various visual artifacts that become especially noticeable near color boundaries due to the non-ideal conditions of the upper frequencies of the QAM color difference signal. The visual artifacts include discoloration and misalignment between color components, such as RGB or PrPb, near edges of the color transition. One particular area in television transmissions where the distortion is especially troublesome is at the edge of a human face, where the distortion causes an unnatural skin tone to be produced, that is easily observed by a video viewer. Because of the particular frequency spectrum, the color misalignment is most visible in the green-to-magenta transition of the radio frequency (RF) color bar signal. The misalignment of the color components results in the appearance of an intermediate color, such as red, for example, along an edge of the transition.
A particular source for this type of distortion is derived from cross-talk between two color difference signals represented in the signals U and V of a QAM decoding. The attenuation of a portion of the upper side band of the U and V color difference signals prevents the proper reconstruction of the video image in high frequency regions, such as are typically found in edge transitions.
FIG. 1 illustrates a situation where the frequency spectrum of two color difference signals U and V do not occupy the same bandwidth. The U and V frequency spectrums illustrated in FIG. 1 provide a spectrum V that has a slightly smaller bandwidth than the spectrum for signal U. Portions X and Y of the frequency spectrum in FIG. 1 illustrate the frequency attenuation of the tuner IF stage on the base band chroma. The frequency attenuation in regions X and Y contribute to distortion based on cross talk between the two color difference signals U and V. It would be desirable to provide a decoding compensation for the distortion and reduce or eliminate the visual artifacts that result from such a distortion.
One technique for reducing the distortion produced in a mistuned IF stage is to filter or compensate the composite signals in the QAM prior to decoding. According to this technique, a compensation is applied to diminish the cross-talk and distortion of the composite signal, often according to user-selectable frequency responses. For example, the compensation may be chosen to be flat, that is no compensation, 6 dB/octave, 12 dB/octave and applied to the IF stage to produce compensation for the distortion and cross-talk between two color difference signals. The compensation of 10 dB/MHz is used in the sequential color with memory (SECAM) television standard signals that are decoded using surface acoustic wave (SAW) filters specified for the phase alternating line (PAL) television reception standard.
Although the type of compensation described above does tend to reduce the effects of visible artifacts in a television signal slightly, the artifacts are still noticeably present. The persistence of the artifacts is due to the fact that the compensation is applied to the composite signal that includes the cross-talk and distortion information in a combined or coded state. The compensation does not separately address the sources of the problems associated with the visual artifacts that must be present in each channel. In addition to providing a general compensation to the overall composite signal, this approach provides a limited flexibility due to its general applicability for different types of tuners, and does not take into account tuners that have excessive and/or atypical distortion characteristics. An illustration of the compensation impact provided according to this conventional technique is illustrated in FIG. 2.
It would be desirable to provide a compensation for the IF stage of a QAM decoder that produces a metacompensation for the effects of distortion related to cross-talk between two color different signals. It would also be desirable to reduce or eliminate the resulting visual artifacts produced by the distortion and the loss of part of an upper frequency side band related to the QAM signal in a mistuned IF stage.
According to the National Television System Committee (NTSC) standard for television signal transmission, each channel is allotted six MHz, which includes four MHz for video and two MHz for audio. Typically, video transmissions are designed to be as wide as possible within the four MHz band allotted, without producing significant distortion in the video signal. In practice, however, distortion often occurs in the video signal, because the signal approaches the limit of the four MHz video band.
In QAM signals, a U and a V channel are provided that are ideally uncorrelated and orthogonal to each other. However, this ideal situation is only true if the side bands of the frequency shifted signal are symmetrical and without distortion. If one side band does become distorted, cross-talk interference or noise is observed in one or the other or both channels. In this instance, the U and V signals are no longer orthogonal. An example of sideband distortion is illustrated in FIG. 1
In the formation of the shifted frequency side bands, several different factors can contribute to distortion or the loss of symmetry in the side band frequencies. One such source is mistuning of an IF stage, which may tend to cause attenuation in the upper side band frequencies. The same result may occur from the use of a surface acoustic wave (SAW) filter that has a poor response characteristic. Often, a sharp transition in frequency in the input video signal produces cross-talk between high contrast areas that causes a fuzziness or “halo” around the sharply changing video features. These sharp transitions are often observed in an area sharply divided in contrast, as is often the case in the area surrounding a human face with a sharply contrasting background. The application of compensation in the composite stage of the decoder tends to reduce only the intensity of the halo, and does not substantially operate to reduce the effect or eliminate it altogether.
It would be desirable to reduce or eliminate the effect of distortion or cross-talk resulting in fuzziness or “halo”type characteristics in sharp transition features of a video signal.